The conditions affecting the irreversible inhibition of catalase by 3-amino-1:2:4-triazole (AT) were described by Margoliash and Novogrodsky. Dialysis of a catalase solution at 37 degrees vs. AT alone showed a slow decrease in activity (about 50 percent in 18 hours). However, if the dialysis solution also contained H2O2 the inactivation was almost complete within 2 hours. To determine if other enzymes which have H2O2 as substrate are affected similarly, human myeloperoxidase (MPO) and horse radish peroxidase (HPO) were each dialyzed vs. 1) 40 mM AT and 2) 40 mM AT-5mH H2O2 at 25 degrees. In the first case no loss in the activity of either peroxidase was observed; while, in the second case the MPO was almost totally inactivated within 2 hours (50 percent in about 1 hour). This inactivation was concomitant with the production of a brown color in the dialysis sack followed by the formation of a brown precipitate. The inactivation of HPO, however, was somewhat slower (50 percent in 2 hours) with no precipitation occuring. This inactivation of the peroxidases may be occuring via a mechanism similar to that for catalse; i.e., the formation of a specific AT modified histidine residue.